Take the example of the Lich upthread: what visual/behavioral cue tips you off to not go for a Con save?
Or dragons: if you haven't read the MM, what clue would suggest they are weaker at one save than another?
I agree that low-level monsters tend to broadcast their strengths and weaknesses, but we're talking about Tier IV, when Wizards are supposedly so much better than Fighters. And I don't think demigod-like abyssals and aberrations and other exotic adversaries are going to broadcast their weaknesses. That frail old crone might have a Strength of 22. That shambling pile of rocks might be a high-level caster (or caster equivalent...they aren't actually casters anymore, right?). That unwieldy blob of goo might have lightning fast reflexes.
After a few rounds of combat, sure, maybe some of these things become more apparent.
And I think DMs should be trying to challenge players at that stage in the game by creating adversaries with non-obvious weaknesses. I don't think anybody would disagree that it's not just fair, but fun and part of the game, to create terrain that challenges the melee types to do something other than close and stab, and prevents the archers from hiding between easy shots. A similar thing should be done for the casters. You don't need to withhold "obvious and relevant information", you just have to make the relevant information non-obvious. That doesn't mean it's necessarily going to be completely opaque, or a 1/6 chance of getting it right. But 90%? That's just handing casters an unnecessary gift.
As I said upthread, if you're facing a lich and you have the option you don't target any save. You just make sure to counterspell.
Dragons obviously have high strength and constitution. Their accumen is also called out in stories, therefore you can expect them to have high mental stats. Therefore, Dexterity.
Have you actually run a high level 5e campaign? I've run several, and played in a few as well. And, this being D&D, there's a lot of combat. Sure, the players will probably face a higher percentage of one-off unique creatures at levels 11+ than before, but there will still be plenty of non-unique creatures that they face multiples of multiple times.
Sure, I can put a belt of storm giant strength on a powerful kobold mage and trick the players. Once. Otherwise they're going to be awash in belts of storm giant strength and that would take the game in the direction of absurdity.
Most of the time, even at high level, they're going to be facing non-unique foes. Let's say I'm running an against the giants style adventure and the players are invading the fortress of a power hungry fire giant earl. Right off the bat the players can probably guess that they shouldn't target strength or constitution, and not to use fire. So, dexterity or a mental stat. And cracking open my MM, would you know it? Dex +3, Int +0, Wis +2, Cha +1. Dexterity is higher than I expected, but against a save DC between 17 to the low 20s, any given fire giant has the odds stacked against them making that save. And that's assuming you're not using abilities that can further stack the odds in your favor like Portent and Silvery Barbs.
And if you're really unsure, or think it has all good saves, you still have the option to either use a spell with an attack roll (thanks to spell attacks, wizards are no worse than the average fighter in this department), buff your allies, or use a spell that doesn't allow saves (Forcecage, Reverse Gravity, etc).
